Reactive sputter deposition is widely used for growing metal oxide coatings. Sputter deposition involves non-equilibrium layer growth at a solid-vapor interface. For a given metal-oxygen system, thermodynamics predicts the phases that should form in the absence of kinetic barriers to growth. Kinetics, however, control the material that is actually formed. For this reason, it is possible to grow high melting point oxides at low temperatures and metastable phases not attainable in a bulk material.
Protective coatings formed by sputter deposition are well known and have been used in a variety of applications. For example, Ito et al. U.S. Pat. No. 5,192,410 describes a laminate wherein a first layer may be a nitride or carbide of zirconium and a second layer made of a transparent ceramic such as aluminum oxide. Mroczkowski U.S. Pat. No. 4,904,542 describes alternating metallic and ceramic layers, wherein the metallic layer can be zirconium and the ceramic material can be a nitride, carbide or oxide of zirconium. In the multilayer structure of Dietrich et al. U.S. Pat. No. 4,919,778, zirconium oxide and aluminum oxide are mentioned as alternatives for the first and fifth layers. In each case, the characteristics and physical properties depend not only on the chemical composition of the successive layers, but also on the crystal structure and thickness of the layers.
The zirconia-alumina materials system model for a transformation-toughened ceramic is well known. See, Science and Technology of Zirconia I, Advances in Ceramics, Vol. 3 (edited by A. H. Heuer and L. W. Hobbs, American Ceramic Society, 1981) and Science and Technology of Zirconia II, Advances in Ceramics, Vol. 12 (edited by N. Claussen, M. Ruhle, and A. H. Heuer, American Ceramic Society, 1984). In a bulk composite, retained t-ZrO.sub.2 particles are incorporated into an alumina matrix. The high temperature forming processes used to fabricate bulk composites preclude using other high elastic modulus ceramics for matrix materials in which an adverse reaction with zirconia occurs. An example of a potential matrix material is silicon carbide (SIC), with an elastic modulus of 400-440 GPa; see Fabrication of Composites, North-Holland, Amsterdam, 1983 p. 373. However, the high temperature required to densify SiC also results in undesirable zirconium silicate formation.
A laminate of alternating layers of alumina and a ceria-zirconia tetragonal polycrystalline ZrO.sub.2 having thicknesses of around 10,000+ nanometers is described in Marshall, Ceramic Bulletin, Vol. 71, No. 6, 1992, pp. 969-973. While these and other composites such as magnesia-stabilized zirconia having reported toughnesses in the range of 10 to 20 MPa.multidot.m.sup.1/2 are known, no comparable laminate containing substantially pure tetragonal polycrystalline zirconia has yet been developed. In particular, despite the availability of literature concerning transformation toughening of bulk zirconia composites fabricated at high temperatures, no method has been proposed for solving the problem of a high temperature reaction between the matrix and zirconia, or for providing a laminate of alumina and tetragonal zirconia. The present invention addresses these problems as described hereafter.